That doesn’t mean you should take up a coffee habit if you don’t already have one. “It’s a mild stimulant and too much can cause jitters and heart palpitations,” Cording points out. “You want to hit that sweet spot but don’t want to overdo it.” She recommends having less than 400 milligrams of caffeine a day (which translates to about four eight-ounce cups of coffee) to see results.